Enabling/disabling SRIOV via sysfs by echo-ing multiple values
simultaneously:

  # echo 63 > /sys/class/net/ethX/device/sriov_numvfs&
  # echo 63 > /sys/class/net/ethX/device/sriov_numvfs

  # sleep 5

  # echo 0 > /sys/class/net/ethX/device/sriov_numvfs&
  # echo 0 > /sys/class/net/ethX/device/sriov_numvfs

results in the following bug:

  kernel BUG at drivers/pci/iov.c:495!
  invalid opcode: 0000 [#1] SMP
  CPU: 1 PID: 8050 Comm: bash Tainted: G   W   4.9.0-rc7-net-next #2092
  RIP: 0010:[<ffffffff813b1647>]
	    [<ffffffff813b1647>] pci_iov_release+0x57/0x60

  Call Trace:
   [<ffffffff81391726>] pci_release_dev+0x26/0x70
   [<ffffffff8155be6e>] device_release+0x3e/0xb0
   [<ffffffff81365ee7>] kobject_cleanup+0x67/0x180
   [<ffffffff81365d9d>] kobject_put+0x2d/0x60
   [<ffffffff8155bc27>] put_device+0x17/0x20
   [<ffffffff8139c08a>] pci_dev_put+0x1a/0x20
   [<ffffffff8139cb6b>] pci_get_dev_by_id+0x5b/0x90
   [<ffffffff8139cca5>] pci_get_subsys+0x35/0x40
   [<ffffffff8139ccc8>] pci_get_device+0x18/0x20
   [<ffffffff8139ccfb>] pci_get_domain_bus_and_slot+0x2b/0x60
   [<ffffffff813b09e7>] pci_iov_remove_virtfn+0x57/0x180
   [<ffffffff813b0b95>] pci_disable_sriov+0x65/0x140
   [<ffffffffa00a1af7>] ixgbe_disable_sriov+0xc7/0x1d0 [ixgbe]
   [<ffffffffa00a1e9d>] ixgbe_pci_sriov_configure+0x3d/0x170 [ixgbe]
   [<ffffffff8139d28c>] sriov_numvfs_store+0xdc/0x130
  ...
  RIP  [<ffffffff813b1647>] pci_iov_release+0x57/0x60

Use the existing mutex lock to protect each enable/disable operation.
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
CC: Alexander Duyck <alexander.h.duyck@intel.com>